In conventional fuel supply systems, a fuel pump sucks fuel from a fuel tank and supplies the fuel out of the fuel pump into an internal combustion engine. Some fuel supply systems are provided with a pressure regulator to regulate a fuel supply pressure, i.e., a pressure of the fuel at which the fuel is discharged out of the fuel supply system, as disclosed in JP-H05-321783-A, JP-H05-039763-A, JP-H06-129325-A (which has counterparts U.S. Pat. Nos. 5,359,976, 5,471,962, 5,577,482, and EP-0593053-B1, EP-0606106-B1), and JP-2002-310025-A. In general, the pressure regulators incorporated in the fuel supply systems have a construction in which a diaphragm (pressure receiving portion) partitions a fuel pressure regulating chamber from a back pressure chamber. The diaphragm is bulged by forces applied by a fuel pressure in the back pressure chamber and by a fuel pressure in the fuel pressure regulating chamber. Thus, the fuel pressure regulating chamber discharges the fuel in accordance with the fuel pressure in the back pressure chamber and the fuel pressure in the fuel pressure regulating chamber so as to regulate the fuel pressure in the fuel pressure regulating chamber. The fuel pressure in the fuel pressure regulating chamber decreases when the fuel in the fuel pressure regulating chamber is discharged, and then the fuel is supplied from the fuel pump into the fuel pressure regulating pressure of the pressure regulator, so as to regulate the fuel supply pressure of the fuel supply system.
The fuel pressure in the fuel pressure regulating chamber, which is regulated by the bulge of the diaphragm, is referred to as a set pressure of the pressure regulator hereafter. The set pressure of the pressure regulator is determined by the pressure in the back pressure chamber, a ratio between a pressure receiving area on one surface of the diaphragm, which is subjected to the pressure in the back pressure chamber and a pressure receiving area on the other surface of the diaphragm, which is subjected to the fuel pressure in the fuel pressure regulating chamber, etc. In a construction in which an elastic member such as a spring applies a biasing force onto the diaphragm, the set pressure of the pressure regulator is determined also by the biasing force of the elastic member. The pressure introduced into the back pressure chamber of the pressure regulator is: an atmospheric pressure; a negative pressure in an intake pipe (refer to JP-H06-129325-A); any one of the negative pressures in the intake pipe and the atmospheric pressure (refer to JP-H05-039763-A); a fuel pressure regulated by another pressure regulator (refer to JP-2002-310025-A), etc.
In this regard, it is recently demanded to raise the fuel supply pressure of the fuel supply system, due to the following reasons.
Firstly, high fuel supply pressure is necessary to compress and liquefy fuel vapors generated in fuel piping. For example, the fuel vapors are prone to be generated when the fuel pump is started on a condition that a fuel temperature is high. High fuel supply pressure is required especially in this condition.
Next, high fuel supply pressure is necessary to promote atomization of fuel injections into the internal combustion engine. For example, it is necessary to promote the atomization of the fuel especially when the internal combustion engine is driving at heavy load, so as to raise an output power of the internal combustion engine.
Further, it is necessary to promote atomization of fuel injections in order to decrease unburned fuel in emission gas discharged out of the internal combustion engine, and to improve startability of the internal combustion engine in low and high temperature conditions. In order to promote the atomization of fuel injections, high fuel supply pressure, at which the fuel is discharged out of the fuel supply system and supplied to fuel injection valves, is effective, in addition to refinements of the fuel injection valves such as adjustments of shapes of injection holes, etc.
When the fuel supply pressure is raised as described above, however, some disadvantages occur. For example, an operating current of the fuel pump rises and an alternator is subjected to heavy load. This decreases fuel efficiency of the vehicle, and shortens a useful life of brushes of a motor for driving the fuel pump, to decrease endurance of the fuel supply system.
In order to raise fuel supply pressure of the fuel supply system, it is necessary to raise the set pressure of the pressure regulator.
In order to raise the set pressure of the pressure regulator, the elastic member such as a spring is upsized to increase the biasing force applied onto the diaphragm, for example. However, the pressure regulator becomes bulky when the elastic member is upsized.
In this regard, JP-2002-310025-A discloses a fuel supply system in which the pressure in the back pressure chamber of a first pressure regulator, which is for regulating the fuel supply pressure of the fuel supply system, is regulated by a second pressure regulator, which is for regulating the back pressure of the first pressure regulator. By setting the set pressure of the second pressure regulator at a high value, it is possible to raise the set pressure of the first pressure regulator without upsizing the pressure regulators.
In the fuel supply system disclosed in JP-2002-310025-A, however, a pressure of excessive fuel, which is discharged out of the first pressure regulator, is adjusted and introduced into the back pressure chamber of the second pressure regulator. In this construction, a quantity of the excessive fuel, which is discharged out of the first pressure regulator in accordance with a fuel consumption quantity of the internal combustion engine, i.e., a quantity of the fuel, which is introduced into the back pressure chamber of the first pressure regulator after its pressure is regulated by the second pressure regulator, is not stable. As a result, the fuel pressure in the back pressure chamber of the first pressure regulator fluctuates depending on the fuel consumption quantity of the internal combustion engine. This causes instability of the set pressure of the first pressure regulator, i.e., instability in the fuel supply pressure of the fuel supply system, which is regulated by the first pressure regulator.